1. Field of the Invention
The present invention relates generally to an underwater sounding apparatus for detecting a fish school by transmitting an ultrasonic signal and forming a receiving beam and, more particularly, to an underwater sounding apparatus capable of calculating fish school information such as the number of fish in the fish school, the volume of a fish school and the backscattering strength of a single fish.
2. Description of the Related Art
Acoustic sounding apparatuses, such as fish-finding echo sounders and scanning sonar, are conventionally used as means for detecting underwater fish schools (refer to Japanese Examined Patent Publication No. 1973-26299 and Japanese Unexamined Patent Publication No. 2003-202370, for example). Installed on a ship, a fish-finding echo sounder transmits a beam of ultrasonic sounding waves (acoustic waves) vertically beneath the ship's hull to search for and detect fish schools existing below the ship. A scanning sonar, also installed on a ship, transmits a beam of acoustic waves into underwater areas surrounding the ship to scan through the surrounding areas and detect fish schools present in these areas. A result of search carried out by such acoustic sounding apparatus is presented as a scanned image in the form of a specific cross section of a detected fish school. From a fisherman's point of view, however, merely displaying the scanned image of a detected fish school in the form of a specific cross section is not very helpful. It is preferable that the result of search be displayed in a manner that enables the fisherman to also recognize the number of individual fish constituting the fish school or the volume of the fish school. With such information it is possible to improve the fishing operation efficiency. This is particularly important for commercial fisherman who increasingly face pressures for increased efficiency.
To achieve this objective, a Japanese Unexamined Patent Publication No. 2003-202370, for example, proposes a scanning sonar which offers a choice of a horizontal scan mode, in which an acoustic sounding beam is horizontally steered at a specific tilt angle to scan through a full-circle area around a ship and a vertical scan mode, in which the acoustic sounding beam is vertically steered to scan an underwater situation in a generally vertical fan-shaped cross sectional area. This scanning sonar presents one or both of scanned images obtained in the horizontal scan mode and the vertical scan mode. This dual-mode presentation enables a fisherman to recognize the shape of a fish school in its entirety and estimate the number of individual fish constituting the fish school or the volume of the fish school, for instance. By comparison, a fish-finding echo sounder does not allow the fisherman to recognize the shape of a fish school in its entirety, because the fish-finding echo sounder transmits an acoustic sounding beam vertically downward from the ship.
Even with the aforementioned scanning sonar which presents the scanned images obtained in the horizontal scan mode and the vertical scan mode, however, it is generally not easy for a fisherman to manually recognize the shape of an entire fish school, for example, from a relationship between the horizontal and vertical mode images, but this skill of recognition greatly depends on the experience of individual fishermen. Furthermore, if fish school information is to be estimated based on the horizontal and vertical mode images, it is quite likely that different fishermen will differently judge the displayed images, producing considerable variations in estimated data. In other words, conventional systems merely provide qualitative, estimated values of the fish school information and other data and fail to provide quantitative measures.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.